Apoptosis is a tightly regulated form of cell death that removes excess or unwanted cells from organisms. Cytochrome c plays a critical role in many apoptotic cascades. When mitochondria receive an apoptotic signal, cytochrome c is released from the mitochondrial intermembrane space into the cytoplasm. Cytoplasmic cytochrome c forms a complex with Apaf-1 and caspase-9 leading to the activation of downstream caspases and subsequent apoptotic cell death. The mechanisms regulating cytochrome c function during apoptosis are poorly understood. In our preliminary studies we investigated the role of nitric oxide (NO) in cytochrome c regulation. NO is an endogenously produced gas that regulates protein function by binding to transition metals or cysteine residues on proteins, a process called nitrosylation. Our preliminary data suggests that cytochrome c is endogenously nitrosylated during Fas-induced apoptosis. Our studies also suggest that cytochrome c nitrosylation increases caspase activation. This is the first demonstration of an endogenous posttranslational modification of cytochrome c during apoptosis. The preliminary findings raise the possibility that cytochrome c nitrosylation is a novel mechanism of apoptosis regulation. This hypothesis will be tested in the proposed studies. In Specific Aim 1 we will determine if cytochrome c is nitrosylated in mitochondria or in the cytoplasm. In Specific Aim 2 we will determine if cytochrome c is nitrosylated on a heme or a cysteine residue. In Specific Aim 3 we will analyze the function of nitrosylated cytochrome c during apoptosis. In Specific Aim 4 we will determine if cytochrome c nitrosylation is a generalized mechanism regulating mitochondria-dependent forms of apoptosis. The results of these studies will determine if cytochrome c nitrosylation is a novel mechanism regulating apoptotic signaling. Ultimately the findings may lead to the development of rational NO-based therapies for diseases associated with dysregulated apoptosis including cancer, autoimmune disease and neurodegeneration. [unreadable] [unreadable]